Rollup merge of rust-lang#25128 - steveklabnik:gh24816, r=alexcrichton

Fixes rust-lang#24816

r? @alexcrichton

Author: Manishearth

src/libstd/fs.rs

@@ -844,6 +844,8 @@ pub fn symlink_metadata<P: AsRef<Path>>(path: P) -> io::Result<Metadata> {
 
 /// Rename a file or directory to a new name.
 ///
+/// This will not work if the new name is on a different mount point.
+///
 /// # Errors
 ///
 /// This function will return an error if the provided `from` doesn't exist, if